WASHINGTON (Reuters) - As he nears the end of a dozen years as director of the FBI, Robert Mueller finds himself defending the agency over its handling of two high-profile cases. It is a familiar spot for the low-key ex-Marine.

At the request of President Barack Obama, Mueller stayed on for two years beyond the job's 10-year term to help stabilize law enforcement's fight against domestic and international threats to U.S. security. Recent events - the bombing at the Boston Marathon and ricin-laced letters sent to Obama and a U.S. senator - have left Mueller dealing with suggestions that agency missteps may have added to the damage.

Mueller, 68, who is scheduled to leave office in early September, has endured many congressional attacks against his agency's performance. While he is not universally praised on Capitol Hill, he has won enough bipartisan support to be considered a success.

Tellingly, it was a target of the 2001 anthrax letters - Democratic Senator Patrick Leahy of Vermont - who told Mueller at a 2008 hearing that he seriously doubted the findings of the FBI's long and complicated anthrax investigation. But three years later, Leahy as Senate Judiciary Committee chairman helped Mueller win a two-year extension of his term.

The FBI chief gets far milder treatment than some other prominent members of the 0bama administration, such as U.N. Ambassador Susan Rice. She came under heavy fire from Republicans for remarks she made after the September 11, 2012, attack on the U.S. Mission in Benghazi in which four Americans were killed, including Ambassador Chris Stevens.

In Washington, where making enemies is easy, Mueller wins high scores from members of Congress for his competence, even as he guided a vast expansion of FBI powers since the hijacked plane attacks of September 11, 2001.

"I believe he is well liked, even though I find fault with a lot of his policies, or how he does things," Senator Charles Grassley, the top Republican on the Judiciary Committee, said in an interview. "I think he's well intentioned ... it's kind of difficult for me to criticize him even when there is a screw-up," Grassley added.

This month, the FBI has faced fresh assaults over its failure to spot the potential danger from Tamerlan Tsarnaev, a suspect in the April 15 Boston Marathon bombings, after Russia asked the bureau to investigate him two years ago. Tamerlan Tsarnaev died in a police shootout and his brother, Dzhokhar Tsarnaev, has been charged in the bombings that killed three people and injured more than 200.

Two senior Republican lawmakers complained Tamerlan Tsarnaev was yet another in a series of cases in which a person investigated by the agency had later taken part in attacks.

Soon after the Boston bombings, the FBI accused an Elvis impersonator of sending letters containing ricin to Obama and other officials, only to quickly drop the charges for a lack of evidence.

Mueller and his leadership team have briefed members of Congress about the cases, but further inquiries are likely.

"I think he is ready to go home. He has had 12 years," said Dutch Ruppersberger, the top Democrat on the House of Representatives Intelligence Committee. He praised Mueller, who was first nominated to the job by a Republican president, George W. Bush.

"He has had some major issues to confront, including 9/11, then Russian and China threats, cyber threats," he said.

Some lawmakers say that despite the Boston blasts, the FBI thwarted other plots in recent years, including one to bomb New York's subway system. They also note that there was an arrest within days of the Boston bombing.

"I thought they (the FBI) did fabulous in getting to the bottom of the Boston bombing, but as great as that was, it was embarrassing to bring in a guy who had nothing to do with the ricin mailings," said Representative Louie Gohmert, a Texas Republican who serves on the Judiciary Committee, which has oversight of the FBI.

Mueller's office did not comment, although Mueller has praised the work of the FBI and other law enforcement agencies for the arrest of the Boston bombing suspect.

LOW PROFILE?

Some think Mueller's secret to success has been keeping a low profile in a town where many people are constantly angling to get attention. Mueller, while doing a lot of closed-door briefings for leaders, does not talk much to the media.

"I think he has been successful because he hasn't been a press hog," Grassley said.

Mueller, a former Justice Department official, was nominated for the top FBI job in 2001 and took office days before the September 11 attacks on New York and Washington.

When Obama asked Mueller to stay on, some Senate Republicans initially balked. One, Senator Rand Paul of Kentucky, came around after Mueller met with him to discuss the bureau's handling of a Kentucky case.

Mueller fielded heavy criticism from his earliest days in office.

After the 9/11 attacks, politicians from both parties questioned how the FBI had missed several warning signs of the hijackings, such as a memo from a Phoenix agent about Middle Eastern men taking flight lessons.

The anthrax case led to criticism because the bureau focused for so long on the wrong man, a mistake that ultimately cost the government millions of dollars in settling a lawsuit.

The FBI finally concluded that Army scientist Bruce Ivans was responsible for the anthrax attacks, but he killed himself before charges were brought.

In 2008, Leahy, then the Democratic chairman of the Judiciary Committee, told Mueller he did not believe the FBI finding that Ivans was solely responsible. Leahy said he thought others had to be involved.

Mueller reorganized the FBI to expand its analytical capability and improve its intelligence collection. He also beefed up its bioterrorism capabilities.

He has been praised for resisting some parts of the expansion of domestic surveillance under Bush.

Grassley said Mueller had not always been as responsive as possible to the senator's letters or "as protective of whistleblowers as he should be."

But he said there should not be another extension as director for Mueller, no matter how well liked he may be.

The purpose of the law limiting the director's term "is to make sure we don't get into this J. Edgar Hoover syndrome, that one guy is so indispensable," Grassley said, referring to the former FBI director of almost four decades. "We don't want to get caught in that syndrome again."

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